This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2000-255624 filed on Aug. 25, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to a brake control device for a vehicle. More particularly, the present invention pertains to a vehicle brake control device for generating a master cylinder hydraulic pressure in a master cylinder in response to a brake pedal operational force, and to a vehicle brake control device for automatically increasing the master cylinder hydraulic pressure generated in the master cylinder to assist the brake pedal operation.
A known brake control device for a vehicle is disclosed in a German Patent Laid-Open Publication No. 197 03 776 A1. This known brake control device includes a hydraulic pressure generating device and a hydraulic pressure increasing device. The hydraulic pressure generating device activates a master cylinder to generate a master cylinder hydraulic pressure in response to a brake pedal operational force. The hydraulic pressure increasing device introduces a servo pressure to a pressure chamber included in the hydraulic pressure generating device. The master cylinder hydraulic pressure is automatically increased by the hydraulic pressure increasing device and other units to assist the brake pedal operation.
In this way, the master cylinder hydraulic pressure is automatically increased with a constant amplification ratio by the hydraulic pressure increasing device in proportion to the brake pedal operational force. Therefore, for example, if an anti-skid control is being applied to a vehicle driving on a slippery road (hereinafter, referred to as a road with a low coefficient of friction) having a relatively small coefficient of friction, when this automatic pressure increasing operation is applied to the master cylinder hydraulic pressure, the brake pressure in each wheel brake cylinder is controlled on the basis of the increased master cylinder hydraulic pressure. In this case, the anti-skid control may not be performed in a stable manner because the wheel brake cylinder brake pressure which should be adjusted to become a low value is controlled on the basis of the excessive master cylinder hydraulic pressure. In this case, a pressure differential between the pressurized master cylinder hydraulic pressure and the wheel brake cylinder brake pressure may become large, wherein an undesired noise during performance of the anti-skid control may occur.
Further, when a coefficient of friction (hereinafter, referred to as a coefficient of friction p on a road) is varied with the vehicle driving, the pressure increase amount applied to the master cylinder hydraulic pressure by the hydraulic pressure increasing device may exceed a desirable amount thereof in some cases, or may be lower than the desirable amount in other cases, wherein a sufficient braking force may not be applied to each wheel.
A need thus exists for a brake control device for a vehicle that is able to increase the master cylinder hydraulic pressure automatically in a way that improves the operation of the anti-skid control in response to the change of the coefficient of friction on the driving road, especially when the vehicle is traveling on a road having a low coefficient of friction xcexc.
A brake control device for a vehicle includes a brake operating member, a wheel, a wheel brake cylinder mounted on the wheel, and a hydraulic pressure generating device. The hydraulic pressure generating device generates a master cylinder hydraulic pressure in a master cylinder in response to the operating force applied to the brake operating member. The brake control device for the vehicle further includes an automatic pressure increasing control device, a braking force control device, a coefficient of friction detecting device, and an automatic pressure increasing restraining device. The automatic pressure increasing control device increases the master cylinder hydraulic pressure automatically in response to the operating force applied to the brake operating member. The braking force control device controls a hydraulic pressure applied to the wheel brake cylinder by using the master cylinder hydraulic pressure. The coefficient of friction detecting device detects a coefficient of friction on the vehicle driving road. The automatic pressure increasing restraining device restrains a pressure increase amount of the master cylinder hydraulic pressure by the automatic pressure increasing control device when an anti-skid control is being performed by the braking force control device when the coefficient of friction detected by the coefficient of friction detecting device is less than a predetermined value.
According to another aspect of the invention, a brake control device for a vehicle includes a brake operating member, a hydraulic pressure generating device for generating a master cylinder hydraulic pressure in a master cylinder in response to an operating force applied to the brake operating member, a plurality of wheels, a plurality of wheel brake cylinders each mounted on a respective one of wheels, a pressure chamber defined in the master cylinder, a pressure increasing device for introducing a servo pressure into the pressure chamber to increases the master cylinder hydraulic pressure with the servo pressure, and an automatic pressure increasing control device for controlling the servo pressure introduced to the pressure chamber by the pressure increasing device and for controlling a pressure increase amount of the master cylinder hydraulic pressure. A braking force control device controls the hydraulic pressure applied to each wheel brake cylinder using the master cylinder hydraulic pressure, and a servo pressure memorizing device memorizes the servo pressure controlled by the automatic pressure increasing control device when an anti-skid control begins to be applied to all of the wheels by the braking force control device. A vehicle deceleration change detecting device detects a vehicle deceleration change occurring after the anti-skid control begins to be applied to all vehicle wheels, and a servo pressure correcting device correct the memorized servo pressure on the basis of the vehicle deceleration change detected by the vehicle deceleration change detecting device. The automatic pressure increasing control device employs the corrected servo pressure as an upper limit of the servo pressure and controls the pressure increase amount of the master cylinder hydraulic pressure while the anti-skid control is being applied to the all of the wheels.